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Multi-Way Conventional loudspeakers with crossovers

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Old 17th August 2010, 08:35 PM   #1
DIFORCE is offline DIFORCE  Australia
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Default Bracing overkill yay or nay ?

Hi , I just had a debate with my friend about bracing a speaker box.
He says that the minimum amount of bracing ( like 1 cross brace at the middle ) is enough , and that adding extra bracing would color the sound because the air is moving in many random directions and reflect off the braces .
While I said that the more bracing the better ( like B&W matrix method ... magico mini bracing method ... ) to achive zero panel resonance .

So what do you think ?
How about something like this for bracing overkill :
Click the image to open in full size.

Last edited by DIFORCE; 17th August 2010 at 08:37 PM.
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Old 17th August 2010, 09:23 PM   #2
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My current project has small boxes (only 5.5L per driver) so does not need extensive bracing, however what I did find was when I did put a brace in (in the only practical place which was right smack in the middle), that I did pick up a fairly significant reflection off the brace back through the driver cone.

Note that this brace (a piece of 1" square oak) was right behind and in the middle of the driver (probably only 7cm from the back of the driver.

So yes if you have braces directly behind your drivers they may cause reflections that come back at you through the speaker, though the layout you have shown I don't think this will be much of a problem. Before gluing them in place though (if possible) I'd do some near filed measurements with and without the braces, and moving their position around). I'd suggest rather than using square braces that you instead use some dowel (broom handle would probably be ok). This should still provide the torsional stiffness you require and should minimise the effects of reflections off the braces

Did you explain to your friend that it was box resonation causing colouration that the braces were intended to stop. I suspect that whilst improper bracing could make things worse, doing the job properly whilst it will change the sound, will be removing colouration, not adding it

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Last edited by wintermute; 17th August 2010 at 09:27 PM.
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Old 17th August 2010, 09:36 PM   #3
DIFORCE is offline DIFORCE  Australia
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the braces appear square only in the sketch , they won't be.
I'm aware of the fact that bracing behind the woofer might cause reflections , I gave the woofer some breathing space behind . ( but in the magico mini the braces are behind the woofer ... like an inch behind ... )

So instead of the early reflections if the braces are dead behind the woofer , there are no drawbacks right ?

I mean look at the magico mini box ... or the B&W matrix that looks like a bee hive right ?
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Old 17th August 2010, 11:07 PM   #4
buggers is offline buggers  United States
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No drawbacks, and make the baffle double thick also...
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Old 17th August 2010, 11:13 PM   #5
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Quote:
Originally Posted by wintermute View Post
My current project has small boxes (only 5.5L per driver) so does not need extensive bracing, however what I did find was when I did put a brace in (in the only practical place which was right smack in the middle), that I did pick up a fairly significant reflection off the brace back through the driver cone.
Tony.
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How does this reflection appear?
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Old 17th August 2010, 11:43 PM   #6
bwaslo is offline bwaslo  United States
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Internal reflections off of braces is unlikely to be an issue, I don't think, at least not compared to vibrating panels.

Also, a little experimenting with braces and an accelerometer (and a meauring system) demonstrates that the cross braces as you show don't actually do nearly as much as would rectangular (yes!) braces attached directly to the walls, to stiffen the wall surfaces as much as possible (not just at a point on the end of the dowels). Try to glue the braces lengthwise to the panels, with ends also attached to walls, so as to leave the smallest expanse of panel unbraced as possible. If the walls were drum heads, you'd want them to be the smallest area drumheads as possible.
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Old 17th August 2010, 11:47 PM   #7
doug20 is offline doug20  United States
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You should build one with bracing and one without then let your friend listen
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Old 17th August 2010, 11:51 PM   #8
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Vibrating panels is a far more serious issue than some very minor internal reflections.

Generally the more bracing the better. Reflections will be minimal - compare the wavelengths invloved with the size of the braces.

I would round any edges off and make sure there's plenty of air flow. You bracing diagram looks very good.
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Old 18th August 2010, 12:04 AM   #9
doug20 is offline doug20  United States
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Isn't making sure the soundwave doesnt bounce back through the woofers more important then over-bracing?

It really does not matter what happens internally because if the sound can not escape it would color anything.
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Old 18th August 2010, 02:11 AM   #10
gedlee is offline gedlee  United States
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Originally Posted by bwaslo View Post
Also, a little experimenting with braces and an accelerometer (and a meauring system) demonstrates that the cross braces as you show don't actually do nearly as much as would rectangular (yes!) braces attached directly to the walls, to stiffen the wall surfaces as much as possible (not just at a point on the end of the dowels). Try to glue the braces lengthwise to the panels, with ends also attached to walls, so as to leave the smallest expanse of panel unbraced as possible. If the walls were drum heads, you'd want them to be the smallest area drumheads as possible.
Bill

I'd have to disagree here. A cross brace will be far more efective than a single brace of the same size attached to the wall. "Fixing" a point on a surface is extremely effective at stiffening a panel. A brace "on" the panel does NOT "fix" a point, it simply stiffens it, but not as much as the cross brace will.

Most material are very strong in compression, but weak in bending. Wood is a classic example. So a given piece of bracing will work best in compression (cross bracing), not bending (panel bracing).

If you have experimental data to the contrary I'd love to see it, but any structural engineer will tell you that "cross" bracing is far more effective than panel bracing. Just look at any bridge! The entire thing is cross braced.
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